Many gas streams such as natural gas streams comprising normally gaseous hydrocarbons such as methane, ethane, propane, etc., synthesis gas streams, etc., are contaminated with sulfur-containing acid gases such as carbon dioxide, hydrogen sulfide, sulfur dioxide, mercaptans, etc. It is important to remove the acid gas impurities from such gas streams in order to enhance their utility and also to avoid environmental pollution. A commonly used method for removing acid gases is a solvent extraction process wherein a lean solvent which has absorption capacity for one or more of the acid gas contaminants is brought into counter current contact with a gas stream to be treated on a continuous basis in an absorption zone. The at least partially purified gas stream is discharged from the absorption zone for further processing or venting to the atmosphere while the solvent, rich in absorbed gases, is discharged from the absorption zone to a regeneration zone where the acid gases are removed from the rich solvent solution to thereby provide a lean solution of solvent for recycle to the absorption zone and a stream of desorbed sulfur-containing contaminants. Such a process is shown, for example in Appl et al. U.S. Pat. No. 4,336,233.
The desorbed stream of carbon dioxide can be used, for example, for enhanced oil recovery.
A wide variety of solvents have been proposed for processes of this nature, such as sulpholanes, alkanolamines, heterocyclic nitrogen-containing compounds such as piperazines and morpholines, etc. The particular solvent that is used in a particular situation is normally selected on the basis of the nature of the gas stream, the degree of contamination of the gas stream to be treated, the pressure of the gas stream and the downstream processing of the gas stream.
Aqueous N-methyl diethanolamine (MDEA) is a preferred gas treating agent for the bulk removal of carbon dioxide from natural gas, associative gas derived from carbon dioxide flooding, coke-oven gases and synthesis gases of any origin. The primary advantage of the MDEA based absorbent stems from the fact that aqueous MDEA can absorb as much as one mole of carbon dioxide per mole of amine whereas common aminoalcohols such as monoethanolamine, diethanolamine, .beta.,.beta.'-hydroxylaminoethyl ether and diisopropanolamine can absorb not much beyond 0.5 mole/mole of amine. Furthermore, under a high carbon dioxide loading condition, the aqueous MDEA absorbent can be regenerated by adiabatic flashing when pressure is reduced. These features contribute to the overall energy efficiency of gas treating processes based on aqueous MDEA.
The energy efficiency and equipment costs of gas treating processes can be further improved if the total amine concentration of the absorbent can be increased. A factor limiting how much MDEA concentration can be increased is that at a certain point any further increase in MDEA concentration will result in a counterproductive absorption rate decrease. The prior art discloses that morpholine, monomethyl ethanolamine and piperazines can be used to increase the total base in the MDEA based absorbent.
Although the results that have heretofore been obtained with N-methyldiethanolamine have been encouraging, they have not been entirely satisfactory and there is a need for improvement.